incoming - packet from outside to this host
outgoing - packet from this host to outside
forwarding - host acting as a router, packet passthrough this host from one interface to another.
https://www.digitalocean.com/community/tutorials/a-deep-dive-into-iptables-and-netfilter-architecture
http://content.hccfl.edu/pollock/AUnixSec/IptablesOverview.htm
http://www.adminsehow.com/2011/09/iptables-packet-traverse-map/
http://stuffphilwrites.com/wp-content/uploads/2014/09/FW-IDS-iptables-Flowchart-2014-09-25.png
ftp://ftp.shorewall.net/pub/shorewall/misc/netfilterflow.pdf
